If i am developing on my particle and and then send it to a customer? What is the easiest way for him to setup his network profile on particle? Without creating a particle account and register it to himself? Or do i have to make a program that check if it is connected to the cloud or not? If not then the customer can connect thru serial and setup the wifi?

Easy for you: Clear the WiFi credentials, then the device will come up in Listening Mode, but the customer would need either a serial terminal (slightly “complicated” on Windows since drivers are needed) or CLI (can become rather complicated to setup).

Easy for the customer: Clear credentials and provide an app/websetup that connects to the Phonons SoftAP

But you must not only think of first time setup but also how to allow for the user to change the credentials if they decide to alter their WiFi.

All this said, depending on your actual use case other options might be more suitable (e.g. with touch screen, add a setup screen with keyboard).